As is well known thrust balancing in a twin spool, axial flow, gas turbine engine is manifested by controlling the leakage of certain seals between the adjacent cavities to attain predetermined pressure levels. Typically, the cavity adjacent the TOBI is one of the cavities that is utilized to maintain this thrust balancing relationship. The TOBI, which sometimes is referred to by other names, serves to direct accelerated cooling air tangentially to the turbine for the purpose of cooling the turbine disks and turbine blades.
TOBIs utilized to inject cooling air to the turbine rotor of a gas turbine engine are aptly described in U.S. Pat. No. 4,822,244 granted to Maier et al on Apr. 18, 1989 entitled "TOBI" and U.S. Pat. No. 4,730,978 granted to Baran, Jr. on Mar. 15, 1988, entitled "Cooling Air Manifold For A Gas Turbine Engine", both of which are assigned to United Technologies Corporation, the assignee common with this patent application. Also U.S. Pat. No. 3,989,410 granted to Ferrari on Nov. 2, 1976 entitled "Labyrinth Seal System" discloses an accelerator (another name for TOBI) that likewise directs cooling air tangentially to the turbine rotor. For further details of the TOBI throughout this patent application reference should be made to these aforementioned patents that are incorporated herein by reference.
In the engine configuration disclosed in the latter patent the thrust balancing is occasioned by maintaining the pressure level in two adjacent cavities to be identical and allowing the flow leakage across the seal between the two cavities to migrate in either direction. Hence the flow in either cavity can migrate to the other depending on the pressure levels in these adjacent cavities. If foreign particles are in the upstream cavity this foreign matter can flow into the downstream cavity and ultimately into the internal cooling passages of the turbine blades. Additionally, this leakage air could eventually mix with TOBI air so as to cause the TOBI air to be at a temperature and pressure that is less than ideal. The addition of means would have to be provided to compensate for this adverse condition which otherwise would result in a deficit in engine operating performance.
In other engine configurations, specialized and expensive, complex seals were employed adjacent the engine shaft in order to attempt to prevent foreign matter from migrating into the turbine blade cooling passages. Of course, with the advent of higher performance engines where the turbine inlet temperature is designed to run hotter than heretofore known designs in order to enhance engine performance the turbine blades are designed with additional holes in the blade to attain a cooler blade to accommodate these higher temperatures. In order to increase the number of cooling holes in these blades it is typical to fabricate the blades with smaller diameter holes, which exhibit propensity to clog and, hence, increase the problems associated with foreign particles carried in the cooling air stream.
We have found that we can obtain the thrust balancing feature and obtain anti-contamination means by incorporating a buffer cavity between two axially spaced seals. The buffer cavity will allow the pressure in the cavity fore of the turbine rotor (FRC) to be increase to a value that is higher than the cavity at the discharge end of the TOBI (TDC) by creating a pressure sink between the two cavities. The pressure in the buffer cavity will be maintained just below the pressure in the TDC and well below the pressure in the FRC by linking it to an even lower pressure area near the engine's flow path with six of the eleven plenums communicating with the dead rim area. This fluid communication with the low pressure area will cause air from the FRC and the TDC to leak into the buffer cavity and out into the engine's flow path. As air continuously leaks out of the TDC and TDC is not polluted.
This invention contemplates that the air that exits through these six plenums will be accelerated and directed tangentially (TOBIed) to provide turbine disk cooling at the dead rim area. The six plenums that are designated to provide buffer flow which are designed identical to the remaining five plenums will include cover plates that are not included in these five plenums so as to cover the openings communicating with the space between the two seals to separate the plenums from the FRC and open a path to the low pressure region. The remaining five plenums will utilize the high pressure FRC's air for increased rim cooling by TOBiing the air through TOBIed holes that are identical to the other six plenums except for the abscense of cover plates. These five plenums will provide the required dump into the flowpath needed to maintain cooling across the first stage turbine rotor.